1. Field of the Invention
The present invention relates to a backlight module used to emit light from behind a liquid crystal display panel or the like and a light source portion used for the backlight module as well as a connection structure of the backlight module and a mounting circuit board.
2. Description of Related Art
Generally, a backlight module for improving visibility is placed behind the liquid crystal display panel. For example, a backlight module is provided to a display portion of portable equipment, such as a cellular phone, so that satisfactory visibility is ensured in circumstances where the brightness of surroundings is insufficient (at night, for example).
As shown in FIG. 8, the backlight module includes an optical waveguide 1, a light source portion 2 that emits light to be incident on the end face of the optical waveguide 1, and a mounting circuit board 3 provided behind the optical waveguide 1 and including a feed circuit to the light source portion 2.
The light source portion 2 is formed by mounting a plurality of light emitting diodes 22 on a mounting surface 21A of a light source portion circuit board 21. The light emitting diodes 22 are of a side view type that emit light in a direction parallel to the mounting surface 21A of the light source portion circuit board 21, and each generates light toward the end face of the optical waveguide 1.
The light source portion circuit board 21 is made of a flexible circuit board. The light source portion circuit board 21 is reversed as being bent at 180°, so that one end portion on which are mounted the light emitting diodes 22 opposes the end face of the optical waveguide 1 while the other end portion is guided onto the surface of the mounting circuit board 3. A wiring pattern (not shown) on the mounting surface 21A of the light source portion circuit board 21 is electrically connected to the mounting circuit board 3 through a connector or a solder joint at the other end portion. This makes it possible to feed the light emitting diodes 22 with electricity from the mounting circuit board 3 through the light source portion circuit board 21.
FIG. 9 is a side view showing a mounting structure of the light emitting diodes 22 with respect to the mounting surface 21A of the light source portion circuit board 21. Lead terminals 22A protrude respectively from the both side surfaces of each light emitting diode 22 in regard to the light emitting direction. The lead terminals 22A are formed so that they are flush with the bottom surface of the light emitting diode 22, and it is the lead terminals 22A that are connected to the wiring pattern (not shown) on the mounting surface 21A through solder joints.
According to the arrangement described above, however, a thickness t of the light source portion 2 cannot be thinner than a sum of a thickness d1 of the light emitting diode 22 and a thickness d2 of the light source portion circuit board 21, d1+d2. Hence, there is a limit to a reduction of the light source portion 2 in thickness, which makes it impossible to reduce the backlight module in thickness and hence the portable equipment in size.
In order to reduce the thickness t of the light source portion 2, there is an attempt to reduce the thickness d2 of the light source portion circuit board 21. However, for example, when the light source portion circuit board 21 is made as thin as 0.15 to 0.2 mm, the rigidity of the light source portion circuit board 21 is deteriorated considerably, and it becomes difficult to handle the light source portion circuit board 21 when the light source portion 2 is assembled. In other words, a special jig has to be used to maintain the shape of the light source portion circuit board 21, which complicates the assembly work of the light source portion 2. As a result, the productivity is reduced.
Further, according to the arrangement described above, the mounting surface 21A of the light source portion circuit board 21 is opposed to the display panel side (above the optical waveguide 1 in FIG. 8), whereas the mounting circuit board 3 is present behind the optical waveguide 1. For this reason, the light source portion circuit board 21 has to be reversed as being bent at 180°. This becomes another reason that makes it impossible to reduce the thickness of the backlight module.
When the mounting circuit board 3 of a large area is used, the light source portion circuit board 21 may be electrically connected to the mounting circuit board 3 without bending the light source portion circuit board 21. However, increasing an area of the mounting circuit board 3 is not acceptable particularly in portable equipment with which a reduction in size is a crucial problem.
In addition, according to the connection structure using a connector or a solder joint, it takes time to assemble a backlight module; moreover, the backlight module cannot be assembled automatically. Hence, there has been a need for another connection structure that can readily connect the mounting circuit board 3 to the light source portion circuit board 21.